1. Field of the Invention
The present invention relates generally to thermoplastic composite structures suitable for surgical implantation as tubular prostheses and to methods of making such implantable tubular prostheses. In a specific aspect, the present invention relates to externally reinforced fluoropolymer composite tubular structures formed by polymer extrusion methods.
2. Description of the Related Art
An ideal implantable tubular prosthesis will closely approximate the physical and physiological characteristics of normal body tissue. A variety of luminal prosthetic materials and structures have been manufactured in attempts to produce just such a prosthesis. While significant progress has been made along many of the parameters defining ideal prostheses, no material or structure has heretofore produced truly ideal performance.
An implantable tubular prosthesis should be biocompatible, resisting degradation or encapsulation by adjacent tissues, and causing neither mutagenic nor allergic responses in the organism. In addition, the prosthesis must be flexible and strong, as well as resistant to tearing, puncture, aneurism and collapse. Among the luminal devices used as conduits for repairing or replacing natural tubular structures are those which serve as conduits for blood, such as endovascular prostheses and vascular grafts. These luminal devices, besides requiring the above-mentioned properties, must also avoid inciting excessive thrombotic responses in the blood they convey.
It has been found that prostheses which effectively avoid the formation of thrombus develop endothelial linings or neointima. The neointima arise through the deposition of adhesion glycoproteins on the interior surface followed by fibrin deposition and endothelial cell migration and growth. The neointimal lining is desirably limited in depth, but commonly exceeds normal epithelial growth limitations (e.g., depth of fibrin layer, confluence of sheet) to, in the case of smaller prostheses, constrict or eliminate the lumen of the synthetic portion of the blood vessel.
Among the measures observed to reduce such vascular compromise, the introduction of pores into the walls of the prosthesis has proven effective. In larger diameter prostheses, such pores are often a product of the woven material used as the prosthetic structure. In smaller vessels, woven materials have been found to be less desirable, and preferred materials include porous fluoropolymers. The porosity of these materials has been found to be a critical factor in their utility as implantable prostheses, since ingrowth of tissue onto the walls of the prostheses directly improves their biocompatibility.
In addition to possessing antithrombogenic characteristics on the internal surface of the tubular prostheses, endovascular conduits and vascular grafts must be sufficiently compliant to withstand the hemodynamics of the body, as well as being resistant to leakage upon implantation. It is very important that the walls of the device possess sufficient radial strength to remain patent (open), thereby allowing proper blood flow, yet also be kink resistant to allow handling and manipulation by the surgeon who implants it.
Fluoropolymer tubes used as flexible implantable devices, e.g., vascular grafts, however, demonstrate a variety of physical limitations directly attributable to the microstructure of the materials and resulting from their method of formation. Most fluoropolymer tubes are extruded, leading to a node-fibril microstructure in the polymer, with the majority of the fibrils oriented in the direction of extrusion, namely axially or longitudinally. Because of this longitudinal bias inherent in the tubes' microstructure, the tubes are relatively strong in the longitudinal or axial dimension. But by the same token the tubes tend to be relatively weak in the lateral or radial dimension. Furthermore, because of the longitudinal orientation of the microfibrils, extruded flexible fluoropolymer tubes designed as implantable prostheses tend to kink, pinch, or collapse when they are bent. The longitudinal orientation of the fibrils produced by extrusion also tends to manifest as a defect when the prosthesis is sutured into place, commonly tearing along the axial dimension at the point where the suture pierces the tube and exerts its tensional effect. Tearing then propagates along the longitudinal direction, causing the loss of structural integrity. According to these physical and mechanical criteria, conventional fluoropolymer prostheses are unacceptably deficient in comparison to normal vascular tissue, which is notably tough but pliable.
Numerous attempts have been made to improve the radial tear strength and to reduce the mechanical deformation of fluoropolymer vascular prostheses. For example, a variety of methods rely on the use of laminated composite materials. Other methods call for controlling the orientation of microfibrils or other microstructure, in order to enhance the radial strength of the extruded fluoropolymer.
Other methods known in the art for improving the toughness and flexibility of fluoropolymer tubes involve adding radial strength by modifying the structure of the prosthetic tube. Such modifications have usually involved adding structural support that is of a magnitude larger than that of the microfibrils. Typically, these methods have employed the use of materials other than fluoropolymers and which have different physical characteristics, particularly greater elasticity. These methods have also often provided a framework of supporting ribs or coils oriented radially or circumferentially rather than longitudinally, additional layers of alternative materials, or both ribs and layers in combination.
For example, a method described in U.S. Pat. No. 4,550,447, provides for extruding a porous fluoropolymer tube and then cutting circumferentially or helically into the external wall of the tube and heating the tube to cause ribs or helices to form. The resulting ribs or helices tend to have a less porous microstructure.
U.S. Pat. No. 5,061,276 describes a vascular graft made from a porous tetrafluoroethylene tubing having a wrapping of elastic fibers applied at various angles oblique to the longitudinal axis and exerting varying tensions upon the tubing.
Materials using helical support measures include those described in U.S. Pat. Nos. 4,306,318 and Re. 31,618. These patent documents describe organic prostheses made from polytetrafluoroethylene tubing and having elastic filaments wrapped around the exterior of the tube in a helical orientation.
U.S. Pat. No. 3,479,670 also describes tubular surgical prostheses. The tubular portions are described as mesh fabric tubes made from tetrafluoroethylene polymer or polyethylene terephthalate. The mesh tubes are wrapped with a low-melting polypropylene monofilamentous helix fused to the outside surface of the tube.
U.S. Pat. No. 4,747,849 describes an esophageal prosthesis. The tubular prosthesis is described as having a flexible inner wall and a rigid outer wall, with a helical filament or yarn of relatively elastic material positioned between the two walls. Materials for the prosthesis include polytetrafluoroethylene polymers, silicone polymers, and, preferably, polyurethane. The prosthesis has a circular cross-section in the central portion, while being elliptical at the orifices at either end.
U.S. Pat. No. 4,850,999 discloses implantable prostheses including a tube and a braided reinforcing component. The hose may be woven or knit or may be of a synthetic resin, including expanded polytetrafluoroethylene. The braid may be a braid of metal filaments or filaments of a synthetic resin. The braid is described as affixed to either the inner or outer surface of the tube by glue or adhesive, or by embedding the braid between two layers of material.
From the previous discussion it is apparent that both conventional textile prostheses as well as PTFE prostheses have respective benefits and disadvantages, but neither offers properties which solve all of the aforementioned problems, and especially the kink, crush and tear resistance properties which are so desirable in synthetic prostheses of the described type.
None of the aforementioned patents disclose unitary implantable prostheses made from a porous fluoropolymer tube reinforced with an external fluoropolymer helical support to provide excellent hemocompatibility and resistance to tearing and deformation. Other measures have generally been needed, requiring the addition of adhesives or heterogeneous reinforcing materials with less desirable characteristics, or requiring precise and complex mechanical manipulation of an extruded tube. These attempts have also failed to produce uniform and integral reinforced prostheses possessing the desired properties described above.
Therefore, it would be a significant advance in the art to overcome the above-described difficulties associated with reinforcing extruded fluoropolymer implantable prostheses, in a manner obviating the use of less biocompatible materials or expensive and difficult mechanical methods of modification of extruded fluoropolymer tubes.
The present invention solves the disadvantages inherent in the prior art by providing a method for manufacturing implantable prostheses from extruded flexible fluoropolymer tubes with fluoropolymer-based reinforcing structures, taking advantage of the excellent biocompatibility of fluoropolymers without compromising the porosity of the prosthesis important for its utility. The present invention also provides implantable prostheses that exhibit superior physical characteristics, such as resistance to suture-induced tears and various types of deformations induced by extraneous stresses generated during implantation as well as in situ.
Accordingly, it is an object of the invention to provide a method of manufacturing an improved, radially reinforced implantable prosthesis.
It is a further object of the invention to provide an improved implantable tubular prosthesis which overcomes the above-described deficiencies of the prior art practice.
Other objects and advantages of the present invention will be more fully apparent from the ensuing disclosure and appended claims.